A380104 Minimal conductors c of complex dihedral normal closures K = L(zeta_3) of pure cubic number fields L = Q(d^1/3), d > 1 cubefree, with elementary bicyclic 3-class group Cl_3(K)=(3,3) and second 3-class group M=Gal(F_3^2(K)/K) of assigned coclass cc(M)=0,1,2,3,...

30, 90, 418, 1626

Offset

1,1

Comments

The coclass cc(M) for one of the fields K with conductor c = a(n) is n-1, and for each field K with conductor c < a(n), the coclass cc(M) is less than n-1. Among the 3-groups M of coclass cc(M)=1, we distinguish the abelian 3-group A=(3,3) by formally putting cc(A)=0, in accordance with the FORMULA. This is a significant difference to quadratic fields, which are firstly uniquely determined by their discriminant, and secondly cannot have an abelian second 3-class group.

Links

Table of n, a(n) for n=1..4.

Siham Aouissi and Daniel C. Mayer, Coclass of the second 3-class group, arXiv:2508.17510 [math.NT], 2025. See p. 12.

Daniel Constantin Mayer, The distribution of second p-class groups on coclass graphs, arXiv:1403.3833 [math.NT], 2014; J. Théor. Nombres Bordeaux 25 (2013), 401-456.

Daniel Constantin Mayer, Magma program "PureCoClass.m" with endless loop

Formula

According to Theorem 3.12 on page 435 of "The distribution of second p-class groups on coclass graphs", the coclass of the group M is given by cc(M)+1=log_3(h_3(E_2)), where h_3(E_2) is the second largest 3-class number among the four unramified cyclic cubic extensions E_1,..,E_4 of the complex dihedral field K, and log_3 denotes the logarithm with respect to the basis 3. An exception is the abelian 3-group A=(3,3) with correct cc(A)=1, where the FORMULA yields cc(A)=0.

Example

We have M abelian for c=30=2*3*5 (a singlet), cc(M)=1 for c=90=2*3^2*5 (two fields in a quartet), cc(M)=2 for c=418=2*11*19, cc(M)=3 for c=1626=2*3*271.

Prog

(Magma) // See Links section.

Crossrefs

Analog of A379524 for real quadratic fields.

Sequence in context: A039462 A370495 A138018 * A004995 A042772 A042770

Adjacent sequences: A380101 A380102 A380103 * A380105 A380106 A380107

Keyword

nonn,hard,more

Author

Daniel Constantin Mayer, Jan 15 2025

Status

approved